Nuclear reactor cores contain a large number of nuclear fuel assemblies. Typically the fuel assemblies are approximately twelve feet long, eight and one-half inches square and weigh about 1,500 pounds. The fuel assemblies have top and bottom end fittings (also referred to as nozzles) and a plurality of longitudinally extending thimble tube members interconnecting top and bottom end fittings to form a skeleton framework. Typically, seven or eight transverse fuel rod support grids, are axially spaced along the thimble tube members. A plurality of fuel rods are supported in an organized array by the grids. Each bottom end fitting includes at opposing corners two positioning holes for interfacing onto core pins positioned on the bottom core plate of the nuclear reactor core so that each fuel assembly can be positioned in a predetermined location in closely spaced relation on the core plate. As many as 180 fuel assemblies are contained in some reactor cores.
The fuel assemblies are closely packed under water in the reactor core and a large amount of heat-transfer surface for removal of the high power produced per unit volume is provided. Spacing of the fuel assemblies can be critical and is based on a predetermined inter-assembly water gap.
The fuel assemblies contained in a reactor core are removed from the core during refueling cycles. Typically, about every eighteen months, one-third of the fuel assemblies will be replaced with new fuel assemblies. In another eighteen months, another one-third will be replaced. This cycle repeats approximately every eighteen months. During refueling, all the fuel assemblies are transferred to a separate fuel assembly storage area, also referred to as a spent fuel pit, located adjacent to the containment building surrounding the reactor core. Each fuel assembly is raised by a crane positioned in the containment building and then transferred in vertical orientation onto an upender. The upender typically is supported on narrow gauge rails. The upender is turned to orient the fuel assembly in a horizontal position, and the upender and fuel assembly thereon are transferred on the rail through a small access opening positioned in the wall of the containment building and into the spent fuel pit adjacent to the reactor core. New fuel assemblies are moved into the spent fuel pit and then transferred together with the other two-thirds of the fuel back into the reactor core by the upender. The crane in the containment building places the fuel assemblies onto the proper core pins.
During reactor shut-down and start-up, the fuel assemblies change temperature. Because the zirconium alloy fuel rods contain heavy uranium pellets, the fuel rods cool and heat more slowly than the other zirconium alloy grids and thimble tube members. This differential cooling rate of the fuel rods from the thimble tube members causes an expedited contraction of the thimble tubes. Subsequent contraction of the fuel rods puts the thimble tubes in compression which results in the fuel assembly becoming bowed. This bow can become as large as 0.500 inch over the twelve foot length of the fuel assembly. This amount of bow makes it difficult to interface the bottom end fitting of the fuel assemblies with the core pins during refueling. In severe cases of fuel assembly bow, adjacent fuel assemblies already positioned on core pins can become damaged as bowed fuel assemblies are reinserted into the reactor core. As the crane moves the bowed fuel assembly into position over the core pins, the bowed fuel assembly sometimes will contact other adjacent fuel assemblies and in some cases, damage the other fuel assemblies.
Additionally, a bowed fuel assembly adversely impacts the performance of the nuclear fuel reactor. The inter-assembly water gap may change resulting in higher thermal neutron flux on the outer fuel rods in the reactor. This could lead to reduced thermal margins for the fuel rod cladding and result in plant operational problems. In U.S. Pat. No. 4,678,625 to Wilson et al., a method of straightening bowed irradiated fuel assemblies is disclosed which teaches determining the length adjustment required for shortening the bowed tubular structural member and forming at least an expansion in the bowed member to shorten the length. This method of straightening bowed fuel assemblies is complex. It is more desirable to provide a more simple and less costly method of straightening the bow in a fuel assembly.